The present invention relates generally to wireless communication systems and, more particularly, to spread spectrum wireless communication systems adapted for use in local area networks (LANs).
Technologies associated with the communication of information have evolved rapidly over the last several decades. For example, over the last two decades wireless communication technologies have transitioned from providing products that were originally viewed as novelty items to providing products which are the fundamental means for mobile communications. Perhaps the most influential of these wireless technologies were cellular telephone systems and products. Cellular technologies emerged to provide a mobile extension to existing wireline communication systems, providing users with ubiquitous coverage using traditional circuit-switched radio paths. Cellular users were able to travel from area to area without losing their wireless connection by handing off their connection from one cellular base station to another. As the demand for cellular products and services grew, the ability of system operators and designers to provide wireless connections to greater numbers of users in a given geographic area became increasingly challenging. This led to the allocation of more frequencies for use in cellular systems by government regulators, as well as to the development of techniques that more efficiently use the allocated bandwidth. One such technique is known as “frequency reuse” which describes an allocation of frequency sets (channels) to cells based on a predetermined pattern. The pattern is designed to provide for a minimum frequency reuse distance, i.e., to separate cells employing the same frequencies by a minimum distance which is determined to meet system specifications for same channel interference.
Wireless local area networks (WLANs), on the other hand, followed a much different evolutionary path than cellular systems. Initial WLAN implementations tended to have architectures designed to provide peer groups with the ability to exchange information on an ad hoc basis, e.g., to connect a personal computer in an office with a printer and a laptop. These small workgroups, in isolated areas, didn't require many of the advanced techniques designed for cellular systems because (a) the small WLAN workgroups generally didn't overlap in their coverage areas and (b) the equipment wasn't nearly as mobile as were cell phones. Additionally, the types of data transfer applications for which WLANs were used, e.g., sending a job to a printer, were relatively delay insensitive. Thus, if interference caused a data transmission error over a WLAN connection, the data could simply be retransmitted with little or no perceived quality degradation. As a result, cellular techniques such as frequency reuse are not employed in today's WLAN systems because intercell interference has not traditionally been considered to be a significant problem in this environment.
Uses of WLANs are, however, undergoing a rapid change. Today's workplace is a more fluid environment with people moving around the workplace and WLAN devices moving more frequently from one place to another. WLAN device groups are being clustered more closely together in densely populated office buildings. Moreover, the advent of the Internet, as well as the addition of video, audio and telephony to more mundane data streams, has increased the sensitivity of WLAN applications to delays introduced by errors and retransmissions. In many cases, the retransmission delay of data packets associated with, e.g., a video clip embedded in a spreadsheet application, can result in annoying gaps in the presentation which are equivalent to the data packets not being transmitted in the first place.
One approach to combat the rise of inter-cell interference related problems in WLAN implementations would be to implement frequency reuse therein. Most of today's WLAN systems operate in accordance with the IEEE 802.11b standard. As will be appreciated by those skilled in the art, IEEE 802.11 specifies that WLAN devices will use one of two spread spectrum access methodologies, specifically either frequency-hopping or code spreading. In frequency hopping systems, a wireless connection between two WLAN units will periodically change frequencies according to a predefined hop sequence. In code spreading (also sometimes referred to as “direct sequence spreading”), the wireless data signal is spread across a relatively wideband channel by, for example, multiplication with a pseudorandom noise (PN) sequence. Another example of a Physical Layer (PHY) code application is the transmission of a particular Orthogonal Frequency Division Multiplexing (OFDM) carrier pattern, as might be conducted using the 802.11a standard.
In either of the last two examples of WLAN systems, each channel requires a relatively wide frequency bandwidth. This limits the number of channels that are available in any given WLAN band. For example, in the 2400 MHz ISM band specified for WLAN usage in the United States, only eleven channels are available for simultaneous use to support different WLAN connections. The numbering system stems from frequency-hopping designations. Of these eleven channels, only three (numbers 1, 7 and 11) do not overlap in frequency when used with broader spread-spectrum (802.11b) spectral occupancies. Thus, the best possible frequency reuse pattern would only involve three different cells, resulting in a reuse distance that Applicants believe will not provide a sufficient protection against interference to enable the quality of service which will be demanded in future WLAN generations. Other modern WLAN-like services, such as the 5 GHz U-NII band with larger spectrum allocations, achieve higher-reuse factors but also tend to display increased susceptibility to interference due to the higher transmission rates used.
Accordingly, it would be desirable to provide techniques in addition to, or as an alternative to, frequency reuse which will reduce interference between closely spaced WLAN implementations.